Optical filters are widely used to control visible and solar energy. Most notably, optical filters have been used as glazings in window technology to control the flow of light and heat into and out of the glazing, according to occupant comfort. Applications include, for example, windows in buildings, vehicles, aircraft, spacecraft and ships. Optical filters have also been used to regulate lighting and heating levels, for glare reduction, and energy load management. Improving the energy efficiency of buildings is a key aspect of reducing energy use and reducing CO2 emissions. The United States alone consumes 100 quads of primary energy annually. Buildings consume about 39% of all energy and 68% of the electricity used in the United States. They are responsible for about 38% of all greenhouse gas (GHG) emissions. Windows are responsible for about 30% of a building's energy loss. As such, windows with improved technology for reducing heat loss and solar heat gain can offer significant benefits and cost savings.
Optical filters have also found application in ophthalmic devices to control the light impacting the eye. Applications include, for example, prescription and non-prescription glasses, goggles, sunglasses, visors, and safety eyewear.
There are a number of technologies that have been used in optical filter applications for dynamically varying the degree of visible light transmittance, including photochromics, electrochromics, liquid crystals, thermochromics, and suspended particle displays.
Photochromics
Photochromics react to light levels automatically by darkening in sunlight and certain bright light conditions, and by spontaneously going clear or less dark indoors or in low light conditions. A well-known application of photochromics is found in the eyeglass lenses made by Transitions Optical Inc. of 9251 Belcher Road, Pinellas Park, Fla., USA, 33782. These lenses employ a photochromic compound (or compounds) embedded in a rigid plastic or glass lens, or a photochromic film applied to a lens. For example, U.S. Pat. No. 6,065,836 describes a photochromic ophthalmic lens with a film adhering to the lens containing a photochromic dye.
Photochromics auto-darken in bright light to reduce the amount of visible light transmitted. Reversion of photochromics from the dark to light state, however, is slow and cannot be controlled manually. Photochromics can also be very temperature dependent and tend to break down on exposure to UV light. As such, photochromics have not proven to be a practical technology for some optical filter applications.
U.S. Pat. Nos. 5,604,626; 5,838,483, and 6,246,505, describe photochromic devices having some degree of user control through electronics. These photochromic devices are based on metal oxide photochromics requiring power to maintain the device in the dark state.
Electrochromics
Electrochromics can be used to dynamically alter the visible light transmission properties of a material through the application of electricity. Electrochromic technology involves applying thin coatings of electrochromic materials to two transparent electrodes and sandwiching an electrolyte material in between. Unlike photochromic technology, electrochromic technology typically requires the user to apply external electrical power to darken. Electrochromic technology is used in auto-dimming automobile mirrors (for example, those made by Gentex Corporation of Zeeland Mo.).
U.S. Pat. No. 6,934,067, describes an electrochromic rear view mirror with a gelled electrochromic material formed between two glass substrates with conductive coatings and a perimeter seal. The electrochromic material darkens and lightens when electricity is applied but will not darken automatically. These electrochromic mirrors respond to changing light conditions by changing the light transmittance properties of the mirror through the use of electronic light sensors and electronic controls. Electric power is required to cause the electrochromic material to go darker.
Another example of electrochromics is in window applications (Sage Electrochromics Inc. of Faribault, Minn.) that incorporate thin coatings applied to one of the glass layers in a window. Application of electricity with the positive lead connected to one electrode causes the window to darken, and application of electricity with the positive lead connected to the other electrode causes the window to lighten. The electrochomic coating that is applied to the glass involves the use of specialized coating processes such as sputtering and chemical vapor deposition. This often requires a specialized factory or facility requiring the glass to be shipped to one central factory for the coating process to be performed, and then shipped out to wherever they will be used. As such, windows made using electrochromic technology can be quite expensive.
Electrochromics have also been used in ophthalmic devices. For example, ChromoGenics of Uppsala, Sweden makes an “electrochromic foil” for use in motorcycle helmet visors and other products by making a multi-layer electrochromic device between two plastic films. Relatively low DC voltages are used for switching the electrochromics from one state to another but power is typically required to maintain the electrochromic device in the dark state.
Liquid Crystals
Liquid crystal filters are manufactured by sandwiching a liquid crystal material between two transparent electrodes. When an electric field is applied between the electrodes, the liquid crystals align in a certain orientation to allow light to pass through the filter. In the absence of a field, the liquid crystals have a random orientation and scatter the light, appearing translucent to an observer. Although some light is allowed to pass through in this state, the optical filter will appear translucent or almost opaque and will not be optically clear. This makes liquid crystals only useful for applications such as for privacy glass when being able to see through the optical filter in the dark state is not desirable. Relatively high voltages required for switching the liquid crystals, expensive manufacturing costs, and temperature dependency have limited the application of liquid crystal technology to indoor applications and electronic devices.
U.S. Pat. No. 7,459,189, describes a liquid crystal device that can be used in privacy windows. The technology involves a liquid crystal composite sandwiched between electrodes that permits light to pass in one state and scatters light in another state.
U.S. Pat. No. 7,300,167, describe an adjustably opaque window also based on liquid crystal technology.
Nippon Sheet Glass of Tokyo, Japan, manufactures an optical film made using liquid crystal technology that can change from a translucent to an opaque state with the application of a relatively high (e.g., 120 Volts) AC voltage.
Suspended Particle Displays
Suspended Particle Displays (SPD) involve many small particles suspended in a liquid between two sheets of glass with conducting electrodes. Like liquid crystals, a voltage applied across the electrodes causes the particles to align and light is transmitted. In the absence of a voltage, the particles are randomly distributed and scatter light. The scattering of light means that SPD devices are typically not optically clear in the dark state. SPD devices can also be expensive to manufacture and typically require that the particles be suspended in a liquid so they have sufficient mobility to move. Examples of application of this technology include, U.S. Pat. No. 5,463,491 which describes a filter for windows that comprises an “encapsulated liquid suspension”. U.S. Pat. No. 6,910,729 describes a self-darkening glass based on SPD for providing increased thermal comfort in vehicles.
Thermochromics
Thermochromic filters darken and lighten in response to temperature changes, typically going darker as it gets hotter and as such cannot be manually controlled. An example of thermochromic technology is described in U.S. Pat. No. 6,084,702.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.